6816 Chem. Mater. 2008, 20, 6816–6828

Probing the Structural/Electronic Diversity and Thermal Stability of Various Nanocrystalline Powders of Nitride GaN

Mariusz Drygas,† Zbigniew Olejniczak,‡ Ewa Grzanka,§ Miroslaw M. Bucko,| Robert T. Paine,⊥ and Jerzy F. Janik*,† Faculty of Fuels and Energy and Faculty of Materials Science and Ceramics, AGH UniVersity of Science and Technology, al. Mickiewicza 30, 30-059 Krakow, Poland, Institute of Nuclear Physics, Polish Academy of Sciences, ul. Radzikowskiego 152, 31-342 Krakow, Poland, Institute of High Pressure Physics, Polish Academy of Sciences, ul. Sokolowska 29/37, 01-142 Warszawa, Poland, and Department of Chemistry, UniVersity of New Mexico, Albuquerque, New Mexico 87131 ReceiVed March 5, 2008. ReVised Manuscript ReceiVed July 29, 2008

Nineteen different nanopowders were prepared with four distinct synthesis methods under a wide range of experimental conditions. Variations of synthetic details were aimed at making diverse GaN powders, mainly, from a viewpoint of their structural and thermal stability properties. The supplemental application of powder XRD and 69Ga MAS NMR spectroscopic methods enabled a coherent insight into both the long-range structural order and electronic features, respectively, supporting the specific Knight shift effect displayed in the NMR spectra of the powders. Thermogravimetric measurements under a neutral gas atmosphere were conducted to probe stability ranges of the powders. The combined results support a broad diversity of GaN nanopowders that require supplemental characterization methods to satisfactorily define their complex structural, compositional, and electronic properties evolved during high-temperature syntheses or thermal annealing.

Introduction stock, a limiting factor in successful optimization of the sintering process. The utilization of powder forms of many new materials including semiconducting gallium nitride GaN is far from Powder XRD has been a key method for structure satisfactory despite significant progress in the development elucidation of GaN crystalline powders. However, estima- of relevant synthetic methods.1 One of the attractive ap- tions of even such common parameters as average crystallite plications to consider is powder sintering into mechanically sizes and lattice parameters can be severely misinterpreted robust and machinable ceramics that could be a material of for nanopowders if standard calculation tools that have been 3 choice, for instance, for an economical replacement of GaN derived for microcrystalline systems are applied. In addition, single-crystal supports in optoelectronics. Preliminary reports gallium nitride is known to exhibit a range of defected on the high-temperature high-pressure sintering of nano-GaN hexagonal structures in the nanosized region. Furthermore, powders highlighting selected follow-up applications of the GaN can tolerate a rather undefined extent of nitrogen resulting ceramics have been published elsewhere.2 The deficiency vs the ideal 1/1 Ga/N ratio, with the latter feature crucial conclusion from these efforts based on examination having an undetermined impact on grain crystallinity. All of the GaN ceramic quality was a challenging need for the this makes standard XRD data for nanocrystalline GaN reproducible and unequivocally characterized powder feed- powders highly equivocal. The ambiguity of the XRD data for a limited number of * Corresponding author. E-mail: [email protected]. 69 † such powders has been substantiated by supplemental Ga Faculty of Fuels and Energy, AGH University of Science and Technology. 71 ‡ Institute of Nuclear Physics, Polish Academy of Sciences. or Ga MAS NMR measurements that in principle probe § Institute of High Pressure Physics, Polish Academy of Sciences. | short-range chemical environments of gallium centers in both Faculty of Materials Science and Ceramics, AGH University of Science 4-6 5 and Technology. hexagonal GaN and cubic GaN powders, as stressed ⊥ University of New Mexico. (1) (a) Janik, J. F. Powder Technol. 2005, 152, 118. (b) Gubin, S. P.; Kataeva, N. A.; Khomutov, G. B. Russ. Chem. Bull., Int. Ed. 2005, (3) (a) Palosz, B.; Grzanka, E.; Gierlotka, S.; Stel’makh, S.; Pielaszek, 54 (4), 827. (c) Trindade, T.; O’Brien, P.; Pickett, N. L. Chem. Mater. R.; Lojkowski, W.; Bismayer, U.; Neuefeind, J.; Weber, H. P.; Palosz, 2001, 13, 3843. W. Phase Transitions, Part B 2003, 76 (1-2), 171. (b) Petkov, V.; (2) (a) Stelmakh, S.; Swiderska-Sroda, A.; Kalisz, G.; Gierlotka, S.; Gateshki, M.; Choi, J.; Gillan, E. G.; Ren, Y. J. Mater. Chem. 2005, Grzanka, E.; Palosz, B.; Drygas, M.; Janik, J. F.; Paine, R. T. 15, 4654. (c) Tamas, U.; Gubicza, J. Z. Kristallogr. 2007, 222, 114. Proceedings of the International Conference on Nanoscience and (4) (a) Han, O. H.; Timken, H. K. C.; Oldfield, E. J. Chem. Phys. 1988, Technology, Basel, Switzerland, July 30-Aug 4, 2006; Institute of 89 (10), 6046. (b) Jung, W.-S.; Park, C.; Han, S. Bull. Korean Chem. Physics Publishing: Philadelphia, PA, 2006; Poster 1399. (b) Grzanka, Soc. 2003, 24 (7), 1011. (c) Jung, W.-S. Mater. Lett. 2006, 60, 2954. E.; Stelmakh, S.; Gierlotka, S.; Swiderska-Sroda, A.; Kalisz, G.; Palosz, (d) Schwenzer, B; Hu, J; Seshadri, R,; Keller, S; DenBaars, S. P.; B.; Drygas, M.; Janik, J. F.; Paine, R. T. Presented at the European Mishra, U. K. Chem. Mater. 2004, 16, 5088. (e) Schwenzer, B.; Hu, Powder Diffraction Conference, EPDIC-10, Geneva, Switzerland, Sept J.; Wuc, Y.; Mishra, U. K. Solid State Sci. 2006, 8, 1193. 1-4, 2006; Z. Kristallogr., 2007, 26, MS11-P92. (c) Borysiuk, J.; (5) (a) Yesinowski, J. P.; Purdy, A. P. J. Am. Chem. Soc. 2004, 126, 9166. Caban, P.; Strupinski, W.; Gierlotka, S.; Stelmakh, S.; Janik, J. F. (b) Jung, W.-S.; Han, O. H.; Chae, S.-A. Mater. Chem. Phys. 2006, Cryst. Res. Technol. 2007, 42 (12), 1291. 100, 199.

10.1021/cm800645q CCC: $40.75  2008 American Chemical Society Published on Web 10/16/2008 DiVersity of Stability of Nanocrystalline GaN Powders Chem. Mater., Vol. 20, No. 21, 2008 6817 originally in studies by W.-S. Jung et al.4b,c and B. Schwenzer wders. They were prepared with several processing condi- et al.4d,e In the case of hexagonal GaN, especially for GaN tions by using four different synthesis methods thus providing powders obtained in the 1000-1100 °C range from am- an unparalleled set of materials. Powder XRD and 69Ga MAS monolysis of oxygen-bearing gallium precursors, the well- NMR spectroscopy occasionally supplemented with XPS defined hexagonal pattern for h-GaN in its XRD scan has determinations were used for structural assignments. In this been associated with a usually minor resonance at ca. 330 regard, if the peak resolution is not of major concern ppm in the respective gallium NMR spectrum. In contrast, (significant distances between resonance frequencies), both a major broad NMR line at ca. 400-450 ppm has been linked the 69Ga and 71Ga NMR experiments provide qualitatively by a few researchers to a structurally undefined nitrogen matched and quantitatively comparable results while the deficient phase or impurity (O. H. Han,4a W.-S. Jung,4b,c B. observed peak positions usually vary by a few parts per Schwenzer4d,e). In only one study known to us, a few high- million. Because the resonance frequencies in GaN for the temperature made Ga15N powders have been shown by XRD observed species differ by some plus 100 ppm, even with to display a rather unusual double hexagonal pattern, the significant peak broadness one gets satisfactory data suggesting the existence of two different h-GaN phases.4d,e resolution from the 69Ga NMR experiment. TGA/DTA Some of these powders exhibited a sharp resonance at 330 studies under an UHP helium atmosphere were carried out ppm in the 71Ga MAS NMR spectrum accompanied by a to evaluate powder stabilities at elevated temperatures. For broad resonance with shoulders in the 350-500 ppm range. several samples, residual oxygen contents were also deter- Similarly shaped resonances were also observed in the mined to estimate the completeness of nitridation. respective 15N MAS NMR spectrum. The existence of two hexagonal phases and the presence of two sets of the Experimental Section correlated multinuclear NMR resonances were proposed as an argument for the crystalline (hexagonal) character of the Preparation. Four distinct synthesis routes were applied to make distinct N-deficient phase. GaN powders. (i) A two-stage aerosol-assisted method utilized · x In an alternative breakthrough proposal, the Knight shift aqueous or DMF solutions of gallium Ga(NO3)3 H2O (Strem Chemicals).8 First, an aerosol mist of the solution was due to conduction electrons in a semiconductor has been injected into a ceramic tubular reactor where it mixed with an proposed by Yesinowski and co-workers to account for the ammonia-rich atmosphere at 1000-1100 °C under specific process second high-frequency peak shifting and broadening in the conditions to yield a raw aerosol powder with an average composi- gallium MAS NMR spectra for at least some GaN powders, tion GaxOyNz. This stage was followed by pyrolysis of the raw especially those intentionally doped and/or containing im- powders at a selected temperature in the 900-1000 °C range under purities even at low levels of a few tenths of a percent.5a,6 an ammonia flow, 0.2 L/min, producing gray-yellowish materials. This well-substantiated approach stresses the importance of It is worth noting that the pyrolysis experiments resulted also in electronic factors due to dopant (intentional or unintentional) the formation of substantial quantities of black deposits at the related conduction electrons over purely short-range chemical reactor’s cold exit. In two separate cases, the first stage was carried ° environment effects and is also consistent with the single out under a neutral nitrogen atmosphere at 400 or 1050 C to yield pattern for h-GaN in most of the corresponding XRD spectra. aerosol-generated Ga2O3 as raw powders that were later pyrolyzed in the second stage under an ammonia flow resulting in severely It should not be surprising that the structural ambiguities agglomerated GaN bodies. In one case, the first stage was conducted for GaN powders outlined above have high relevance to their with a dimethylformamide (DMF) solution of gallium nitrate. thermal stabilities. In a majority of synthesis methods, Furthermore, one first-stage experiment was carried out in an especially those based on nitridation of oxygen-bearing Inconel tube reactor to check for a possible impact of metal- gallium precursors, the required pyrolysis temperatures are containing reaction environment on the raw powder generation. One in the 950-1000 °C and higher range, which also happens product powder was additionally cleaned by treatment with a 5% to be a stability threshold for GaN. Additionally, in such a aqueous solution of HF for 20 h. (ii) The nitriding conversions of temperature regime, recrystallization and sublimation phe- commercial (Aldrich) and aerosol-generated Ga2O3 powders (see above) were done in ceramic crucibles under an ammonia flow, nomena are known to reach noticeable levels. It is therefore 0.2 L/min, by setting an appropriate temperature in the 900-975 clear that from the viewpoint of high-temperature GaN °C range and, typically, 6 or 12 h time of pyrolysis. Products were powder applications progressing decomposition phenomena grayish yellow to dark gray powders. (iii) The anaerobic method have to be especially appraised. In this regard, thermogravi- employed the pyrolysis of freshly synthesized gallium imide at 600, metric methods seem to conveniently and adequately address 700, and 975 °C under an ammonia flow of 0.2 L/min as already the powder stability issue.7 published.9 The pyrolyses at 700 and 975 °C were followed by 1 h Herein, we report investigations of structural, electronic, evacuation at the pyrolysis temperature before cooling to ambient and stability properties for a diverse range of GaN nanopo- temperature. The products were yellow to gray powders. (iv) The nitriding pyrolyses of powdered monocrystalline GaAs (a generous gift of Dr. Slupinski, Institute of Experimental (6) (a) Yesinowski, J. P. Phys. Status Solidi C 2005, 2 (7), 2399. (b) Physics, Warsaw University, Warsaw, Poland) were done at 700 Yesinowski, J. P.; Purdy, A. P.; Wu, H.; Spencer, M. G.; Hunting, J.; DiSalvo, F. J. J. Am. Chem. Soc. 2006, 128, 4952. (7) (a) Groh, R.; Gerey, G.; Bartha, L.; Pankove, J. I. Phys. Status Solidi (8) (a) Wood, G. L.; Pruss, E. A.; Paine, R. T. Chem. Mater. 2001, 13, A 1974, 26, 353. (b) Li, J. Y.; Chen, X. L.; Qiao, Z. Y.; Cao, Y. G.; 12. (b) Janik, J. F; Drygas, M.; Stelmakh, S; Grzanka, E.; Pałosz, B.; Lan, Y. C. J. Cryst. Growth 2000, 213, 408. (c) Xiao, H.-D.; Ma, Paine, R. T. Phys. Status Solidi A 2006, 203 (6), 1301. H.-L.; Lin, Z.-J.; Ma, J.; Zong, F.-J.; Zhang, X.-J. Mater. Chem. Phys. (9) (a) Janik, J. F.; Wells, R. L. Chem. Mater. 1996, 8, 2708. (b) Wells, 2007, 106, 5. (d) Unland, J.; Onderka, B.; Davydov, A.; Schmid-Fetzer, R. L.; Janik, J. F.; Gladfelter, W. L.; Coffer, J. L.; Johnson, M. A.; R. J. Cryst. Growth 2003, 256, 33. Steffey, B. D. Mater. Res. Soc. Symp. Proc. 1997, 468, 39. 6818 Chem. Mater., Vol. 20, No. 21, 2008 Drygas et al.

Table 1. Summary of Synthesis Methods and Resulting Gallium Nitride GaN Powdersa (Residual Oxygen Contents for Powders, O, x wt %, Are Shown, If Available) synthesis method

aerosol-assisted: stage 1//stage 2 Ga2O3 nitridation anaerobic GaAs nitridation

A1 (1000/H2O//1000/6); O, 0.4 wt %; gray B1 (comm/900/12); O, 4.2 wt %; l. yellow C1 (600/16); D1 (700/150/slow); A2 (1050/H2O/Inconel//1000/8); gray-beige yellow gray-beige A3 (1050/DMF//975/6); d. gray B2 (comm/950/12); O, 1.7 wt %; gray C2 (700/4/vac); D2 (800/90/slow); A4 (1050/H2O//975/6); O, 1.9 wt %; l. gray yellow-gray gray-beige A5 (1100/H2O//900/6); O, 4.5 wt %; yellow B3 (comm/975/12); O, 1.3 wt %; d. gray C3 (975/4/vac); D3 (800/90/fast); A6 (1100/H2O//950/6); O, 2.9 wt %; yellow gray gray-beige A7 (1100/H2O//1000/6); O, 2.1 wt %; l. gray B4 (aero/400//975/6); gray-beige A8 (1100/H2O//975/6/HF); O, 2.1 wt % B5 (aero/1050//975/6); O, 3.8 wt %; l. gray before HF treatment; l. gray a In parentheses: (i) samples A-aerosol generation temperature (°C)/solvent//pyrolysis temperature (°C)/pyrolysis time (h); the Inconel tube (A2) and HF washing (A8) experiments are indicated; (ii) samples B-comm/(commercial)/pyrolysis temperature (°C)/pyrolysis time (h); aero (aerosol-generated)/ aerosol generation temperature (°C)/pyrolysis temperature (°C)/pyrolysis time (h); (iii) samples C-pyrolysis temperature (°C)//pyrolysis time (h)/vacuum applied during last hour of pyrolysis; (iv) samples D-pyrolysis temperature (°C)/pyrolysis time (h)/relative NH3 flow rate.

°C for 150 h and at 800 °C for 90 h under a NH3 flow. At a given NMR spectrum was measured using the same acquisition parameters pyrolysis temperature, a choice of pyrolysis time and flow rate of as in the normal frequency range. XPS data were determined with NH3 were crucial to complete nitridation; otherwise, some residual a Vacuum System Workshop Ltd. instrument primarily equipped c-GaAs persisted.5b The products were gray-beige tinted powders. withaAlKR X-ray radiation source (1486.6 eV, 200 W) and All GaN powders are summarized in Table 1. The sample names spectral energies were referenced relative to carbon (284.6 eV). include crucial synthesis parameters shown in the parentheses (see Because for the Al-source strong Auger lines appeared to severely Table 1 footnotes). overlap the N 1 s band, a Mg KR (1253.6 eV) radiation source Characterization. All products were characterized by standard was used to record and refine the nitrogen spectrum. TGA/DTA powder XRD spectroscopy (Siemens D5000 or X’Pert Pro Pana- determinations were carried out on the TA-Instruments STA-SDT R ) - ° lytical with Cu K source; 2Θ 20 80 ). Average crystallite sizes 2960 apparatus (disposable Al2O3 crucibles, helium UHP 5N, were evaluated from Scherrer’s equation applying the Rietveld sample mass 50-80 mg, helium purge before determinations at 30 refinement method. For the evaluation, changes of the line profile °C for 1 h, practical temperature range 40-1100 °C, heating rate parameters compared to a standard sample were utilized. Our 10 °C/min). For selected materials, residual oxygen contents were standard was a polycrystalline alumina sintered body with an determined (Saint-Gobain/Carborundum Corp. Boron Nitride, Am- average grain size over 5 µm subjected to stress relief annealing. herst, NY). The profile parameters depend on the instrument settings used for data collection and on the profile function used for the refinement. In our analysis the full Voight function was used to describe the Results and Discussion profile of the measured diffraction lines. The total profile width is Synthesis Methods and Powder Products. Aerosol- a convolution of the Gaussian profile part and of the Lorentzian Assisted Synthesis. The convenient and highly automated profile part and these parts are combined numerically. In such a method, the full width at half-maximum (fwhm) is only one of a aerosol-assisted synthesis method starts with the affordable few fitted parameters. Solid-state MAS NMR spectra of 69Ga nuclei gallium nitrate precursor yielding a unique spherical particle 8 were measured on the APOLLO console (Tecmag) at the magnetic morphology of GaN. In this case, the utilization of the field of 7.05 T produced by the 89 mm bore superconducting oxygen-bearing precursor results in product powders after magnet (Magnex). A Bruker HP-WB high-speed MAS probe pyrolysis in the optimal 975-1000 °C range with residual equipped with the 4 mm zirconia rotor and KEL-F cap was used oxygen contents up to 1-2 wt %, average crystallite size to record the MAS spectra at the spinning rate ranging from 7 to range typically above 25-30 nm, and crystallites shown by 9 kHz. The spectra were measured at 71.925 MHz using a single XRD to have good-quality hexagonal structure.8b By a 2 µs rf pulse, which corresponded to π/4 flip angle in the liquid. suitable adjustment of experimental conditions, oxygen The acquisition delay used in accumulation was 1 s and the typical contents below 1 wt % are attainable. number of acquisitions was equal to 1000. The frequency scale in ppm was referenced to the 69Ga resonance in the 1.1 M solution It is worth noting that after several second-stage pyrolysis of Ga(NO3)3 in heavy water. All peak positions are reported as runs, substantial quantities of a sootlike black deposit are observed shifts not corrected for the second-order quadrupolar shifts. observed at the tube reactor’s cold exit. Figure 1 presents Note that for the purpose of convenient presentation, the spectra from left to right the XRD scan, 69Ga MAS NMR spectrum are displayed in the normalized mode. To identify the spinning in the metallic -Ga range, and XPS Ga 3d band for the sidebands appearing in some spectra, we measured the selected deposit. The XRD data support the complex composition of samples at two spinning rates, i.e., 7 and 9 kHz. Because of limited the material to include mainly gallium oxide -Ga2O3, bandwidths of the MAS probe and the NMR receiver, the signal hexagonal gallium nitride GaN, metallic gallium Ga, and a from metallic gallium had to be searched by a systematic change minor undetermined component with a probable spinel-like of the resonance offset in the NMR console, paralleled by the corresponding retuning of the probe. Once the NMR signal from phase. This should be compared with the NMR evidence + 10 the metallic -Ga was detected at the 0.45% Knight shift (no signal for metallic -Ga (Knight shift at 0.45%); in this case, was detected in the Knight shift range expected for R-Ga),10 the the instrument’s characteristics ruled out a detection of severely broadened resonances for Ga2O3. Finally, the XPS (10) (a) Stroud, J. D.; Segel, S. L. J. Phys. F: Metal Phys. 1975, 5, 1981. spectrum is consistent with the presence of the prevailing (b) Cornell, D. A. Phys. ReV. 1967, 153 (1), 208. Ga2O3, minor GaN, and some metallic Ga. DiVersity of Stability of Nanocrystalline GaN Powders Chem. Mater., Vol. 20, No. 21, 2008 6819

Figure 1. From left to right: XRD pattern, 69Ga MAS NMR spectrum in the -Ga Knight shift range, and XPS Ga 3d band for the black byproduct deposit in aerosol-assisted method. In all likelihood, these deposits condense from the vapor B3 in Table 1 well-illustrate the effect of temperature on phase upon gas stream cooling down at the reactor’s cold the residual O content in the products made from the exit. The occurrence of the practically nonvolatile oxide and commercial source and suggest the 950-975 °C pyrolysis metal at the far end from the crucible suggests their formation range as optimal to yield powders with O contents below 2 from disproportionation of the volatile and unstable gallium wt %. f + suboxide according to 3Ga2O Ga2O3 4Ga, whereas a A different picture emerges from nitriding of Ga2O3 gas phase ammonolysis of the suboxide would yield GaN aerosol powders prepared earlier via aerosol-assisted de- 11 from Ga2O + 2NH3 f 2GaN + H2O + 2H2. In this regard, composition of gallium nitrate solutions (samples B4 and the likely formation of the suboxide would involve the B5). On the basis of the XRD scans (not shown), the reaction between not yet nitrided Ga2O3 in the raw GaxOyNx homemade aerosol Ga2O3 powder prepared at 400 °C was 12 powder and metallic Ga, the latter originating from a nanocrystalline γ-Ga2O3 and the powder prepared at 1050 concurrent at this temperature range decomposition of GaN °C was the common -Ga2O3 polytype. The subsequent or, alternatively, from oxide reduction with hydrogen from nitriding pyrolyses of these powders at 975 °C resulted in ammonia decomposition.13 As a matter of fact, noticeable severely sintered products. It appears that ammonolysis of metallic gallium was sometimes observed on the crucible’s such Ga2O3 powders proceeds through an undesirable ag- bottom after prolonged nitriding conversions at 975-1000 glomeration stage that results in poor nitridation conversions, °C. These observations confirm that complex and competing which is evidenced by higher residual oxygen contents than reactions take place during the nitriding pyrolysis of oxygen- in the comparable cases of commercial Ga2O3 nitridation, bearing gallium precursors and fine process tuning is required e.g., 3.8 vs 1.3 wt % for powders B5 and B3, respectively. to maximize the target reaction. Anaerobic Synthesis. The anaerobic synthesis of GaN from The case of aerosol generation employing the Inconel pyrolysis of gallium imide has the advantage of providing reactor resulted in a small yield of a black powder that by powders with minimal if any oxygen contents and with ample XRD spectroscopy (not shown) contained mostly nanocrys- room for temperature-controlling crystallite sizes in the low talline γ-Ga2O3. The black color of the product also implied nanometer range down to a few nanometers, which is some metallic gallium, thus supporting significant reduction unattainable by other methods in this study.9 Residual C chemistry operating in this system, likely enhanced by the contents (not analyzed) are expected to be very small, very metallic reactor via extensive ammonia decomposition and often below the detection limits of ca. 0.1% of standard increased hydrogen concentrations. Upon subsequent nitrid- combustion methods.9a In this process, very reactive powders ing pyrolysis at 1000 °C, a gray-beige tinted powder, A2, of mostly defected hexagonal GaN polytype are produced was produced. with high specific surface area and attractive adsorption Nitridation of Ga2O3. The straightforward nitridation of properties stemming from surface derivatization with imide/ gallium oxide powder with ammonia at 950-1000 °C also amide/ammonia functionalities.14 To minimize postreaction yields hexagonal GaN powders with up to 2 wt % residual ammonia adsorption, we carried out some preparations in oxygen contents. As typical for nitriding conversion of this study with an additional step of applying a dynamic oxygen containing Ga-precursors, the necessity of high vacuum in the last hour of pyrolysis. temperatures and long pyrolysis times promoted quite Nitridation of GaAs. A specific feature of the nitriding extensive powder losses due to simultaneously occurring conversion of cubic gallium arsenide GaAs in the 700-900 GaN decomposition and sublimation processes. The con- °C range is the production of GaN powder being a mixture densation of black deposits was also observed. Samples B1- of the predominant cubic variety (zinc blende) and the minor hexagonal polytype (defected) in the lowest microcrystalline (11) Balkas, C. M.; Davis, R. F. J. Am. Ceram. Soc. 1996, 79 (9), 2309. (12) (a) Brukl, D. A. Z. Anorg. Allg. Chem. 1932, 203, 23. (b) Frosch, C. J.; Thurmond, C. D. J. Phys. Chem. 1962, 66 (5), 877. (14) Czepirski, L.; Janik, J. F.; Komorowska-Czepirska, E.; Wells, R. L. (13) Butt, D. P.; Park, Y.; Taylor, T. N. J. Nucl. Mater. 1999, 264, 71. Adsorpt. Sci. Technol. 2002, 20 (8), 723. 6820 Chem. Mater., Vol. 20, No. 21, 2008 Drygas et al.

Figure 2. TGA/DTA curves under UHP helium for selected GaN powders.

Table 2. Weight Changes and Characteristic Temperatures in the Heating of GaN Powders under UHP Helium in TGA/DTA Experimentsa weight loss (wt %) weight increase (wt %) temperature of GaN powder below T1 (

a T1, temperature to which initial weight loss continues; T2, temperature from which weight increase starts; Tdecomp, temperature of abrupt GaN decomposition. range.5b Such powders may under unfavorable experimental tively similar, there are remarkable quantitative differences circumstances contain traces of residual GaAs. In this regard, among the powders. The unusually high initial weight loss the importance of the rate of ammonia flow is illustrated by of 3.1 wt % for sample A8 as compared with the losses of the properties of related samples D2 and D3. Here, the the order of 0.1 wt % for powders B3 and C3 is worth noting. formation of the prevailing rare cubic polytype of GaN can In this regard, powder A8 was additionally treated with an be envisioned as resulting from advantageous topochemical aqueous HF solution to remove oxide species and, apparently, conditions set up by the cubic lattice of the GaAs substrate. significant quantities of H2O were concurrently adsorbed and/ Thermal Stabilities of Powders. The TGA/DTA traces or chemisorbed on grain surfaces. Similar powder A4 that under UHP helium for six selected GaN powders are shown was not HF-treated shows a much lower 1.1 wt % loss at in Figure 2 and derived characteristic parameters are this stage. The low weight loss for powder B3 can be linked displayed in Table 2. All materials show an initial weight to the likely low surface area of this highly crystalline loss below T1 (300-730 °C). Subsequently, five of the material and resulting smaller quantities of adsorbed species materials (excluding C2, which shows a continuous weight while for powder C3 the applied high-temperature evacuation loss) show a weight increase beginning at T2 (500-850 °C) after synthesis appears to remove most of the adsorbed that terminates with an abrupt weight decrease around Tdecomp postreaction gases. at 930-1050 °C, the latter corresponding to rapid GaN From the viewpoint of powder stabilities, the TGA- decomposition. The low-temperature weight loss is due determined decomposition temperatures span the range of primarily to release of surface adsorbed postprocessing gases 930-1050 °C. With the exception of powder C2 (anaerobic and possibly some surface reactive groups condensation synthesis, pyrolysis at 700 °C) the remaining powders despite byproducts. The observed slight weight increase at higher the different origin and phase properties (vide infra) show temperatures likely involves an oxidation effect because comparable thermal stabilities with decomposition temper- weight gain is predicted for oxidation of GaN. Apparently, atures in the close 1000-1050 °C range. In this regard, some contaminant oxygen in the UHP helium is responsible powder C2 with Tdecomp at 930 °C (faster weight decrease for this effect. Although the TGA/DTA traces are qualita- due to decomposition already starts at ca. 850 °C) is DiVersity of Stability of Nanocrystalline GaN Powders Chem. Mater., Vol. 20, No. 21, 2008 6821

Figure 3. XRD patterns for GaN powders A1-A8 from aerosol-assisted synthesis.

Table 3. Crystalline Phase Analysis for GaN Powders A1-A8 from Aerosol-Assisted Synthesis (D (hkl), Average Crystallite Size; a and c, Hexagonal Lattice Parameters)

A1 (1000/H2O//1000/6) A2 (1050/H2O/Inconel//1000/8) A3 (1050/DMF//975/6) A4 (1050/H2O//975/6) major h-GaN (ordered): v. sharp sharp sharp sharp D (100) (nm) 190 56 62 29 D (002) (nm) 140 44 50 19 D (101) (nm) 100 39 46 18 a (Å) 3.1893 3.1882 3.1888 3.1881 c (Å) 5.1867 5.1850 5.1878 5.1892 residual c-GaN no no 2.5 wt % no residual G2O3 no verge of detection no no

A5 (1100/H2O//900/6) A6 (1100/H2O//950/6) A7 (1100/H2O//1000/6) A8 (1100/H2O//975/6/HF) major h-GaN (ordered): v. broad broad sharp sharp D (100) (nm) 11 15 27 31 D (002) (nm) 5 7 14 20 D (101) (nm) 7 10 18 21 a (Å) 3.1834 3.1856 3.1872 3.188(2) c (Å) 5.1956 5.1917 5.1860 5.184(2) residual c-GaN no no no no residual G2O3 no no verge of detection no characterized with the smallest average grain size and GaN of 1 atm.7a It is clear that in the 900-1100 °C range pertinent grains displaying defected hexagonal structure while the to our synthesis conditions, both sublimation and decomposi- remaining powders have significantly larger grain sizes (vide tion phenomena may take place enhanced by the nanosize infra). This suggests that the interplay of the somewhat dimensions of the powders.15 correlated grain size and h-GaN structure inhomogeneity Powder XRD Characterization. Figure 3 includes the factors may be primarily responsible for the trend in diffraction patterns for samples A1-A8 from the aerosol- decomposition temperatures with larger grains of usually assisted synthesis and derived structure data are shown in better crystallinity showing overall higher stabilities. Table 3. The major product by XRD in all samples is a At this point, it is worth indicating that the reported GaN relatively well-ordered hexagonal GaN, h-GaN, with a range decomposition temperatures span a wide range from below of average crystallite sizes. The calculated lattice parameters 800 °C (400 °C under H2) to higher than 1100 °C and depend are in reasonable agreement with the values reported for on the applied conditions (vacuum, neutral or reactive gas crystalline h-GaN16 (e.g., ref 16a: a ) 3.189(1) Å, c ) atmosphere, time, specifics of detection technique) and the 5.185(5) Å). Trace amounts of -gallium oxide are found kind of GaN sample.7,13 It is interesting to compare this range only in sample A2 from the unconventional synthesis that with the TGA-derived temperature of 930 °C used for used the Inconel reactor and in sample A7. In sample A3 intentional sublimation of the ball-milled GaN powders7b and (aerosol generation from a DMF solution), small quantities 1120 °C decomposition temperature for microcrystalline 7c ° powders from nitriding of Ga2O3 or 800 C originally (15) Xu, F.; Xie, Y.; Zhang, X.; Zhang, S.; Liu, X.; Xi, W.; Tian, X. AdV. reported as GaN sublimation temperature under the pressure Funct. Mater. 2004, 14 (5), 464. 6822 Chem. Mater., Vol. 20, No. 21, 2008 Drygas et al.

Table 4. Crystalline Phase Analysis for GaN Powders B1-B5 from Ga2O3 Nitridation (D (hkl), Average Crystallite Size; a and c, Hexagonal Lattice Parameters) B1 (comm/900/12) B2 (comm/950/12) B3 (comm/975/12) B4 (aero/400//975/6) B5 (aero/1050//975/6) major h-GaN (ordered): broad sharp v. sharp sharp sharp D (100) (nm) 20 39 84 41 43 D (002) (nm) 10 27 60 18 17 D (101) (nm) 13 28 51 24 25 a (Å) 3.1869 3.1882 3.1893 3.1870 3.1863 c (Å) 5.1923 5.1885 5.1873 5.1886 5.1882 residual G2O3 no verge of detection no verge of detection 7.6 wt % of the cubic GaN polytype are detected. In the latter case, from aqueous solutions, standard times and temperatures) plausible carbothermal reduction/nitridation processes may to obtain powders with the average crystallite sizes signifi- have contributed to the formation of c-GaN. Surprisingly, cantly below 20 nm and simultaneously showing low residual no gallium oxide is detected by XRD in powders A5 and O contents. Note the outstandingly high crystallite sizes A6 that by elemental analysis contain significant quanti- (above 100 nm) for the very ordered h-GaN component in ties of oxygen, i.e., 4.5 and 2.9 wt %, respectively, and powder A1 from the highest applied pyrolysis temperature the patterns misleadingly suggest the exclusive presence of of 1000 °C that also shows the smallest residual O content the nanocrystalline h-GaN phase. The observation should of 0.4 wt %. be traced to a large width of the diffraction peaks for these Figure 4 includes the XRD patterns for samples B1-B5 powders due to small GaN/Ga2O3 crystallite sizes that cause from Ga2O3 nitridation and pertinent structure data are overlapping of the small intensity and broad peaks for the presented in Table 4. As in the case of the chemically similar postulated nano-oxide. This situation can be compared to aerosol-assisted synthesis, all powders appear to be composed the XRD results for well-crystallized powder B5 (vide infra, mainly of the well-crystallized h-GaN. Of the three closely Table 4) where the 3.8 wt % O-content is clearly detected related powders B1-B3 that were pyrolyzed at the increasing in the form of -Ga2O3 and for powder B1 comparable in temperatures of 900, 950, and 975 °C, respectively, only crystal characteristics with powder A6, with the former the pattern for B2 (O, 1.7 wt %) shows some -Ga2O3 showing the 4.2 wt% O-content and no XRD-detected oxide at the detection limits of the method. As previously discussed, phase. A detailed curve-fitting analysis of the broad diffrac- the broad diffraction lines for B1 (O, 4.2 wt %) include tion peaks for powders A5, A6, and B1, however, provides overlapping of the low intensity and broad oxide diffractions solid evidence that the nanocrystalline gallium oxide is by the strong peaks for h-GaN. The decreasing O-contents present in all these cases. On the basis of the comparison of from B1 to B2 to B3 (Table 1) are consistent with improved numerous XRD patterns for nano-GaN including those with nitridation conditions at the highest temperatures in the minimal O contents, in our opinion a simultaneous slight 950-975 °C range. This trend is followed by increased increase of the c lattice parameter in all these powders can crystallite sizes for the powders, i.e., D (101) of 13, 28, and be linked to an averaged bulk/surface effect in small 51 nm, respectively. The worst nitridation circumstances nanocrystallites rather than involving O- for N-substitution effect, i.e., transient gallium oxynitride formation in the bulk phase,17a the latter more likely at high pressures or in thin films phases.17b-f The crystallite sizes calculated for the (002) and (100) diffractions are referred to the crystallite dimensions along the c-axis and perpendicular to it, respectively, indicating a prevailing crystallite shape, whereas the size calculated from the (101) diffraction reflects more of an average grain size characteristics. From the data in Table 3, a platelet-like crystallite morphology is indicated in most cases, being especially visible in the smaller sized powders A4-A7. The powders with the smallest crystallite sizes of GaN estimated from (101), 7 nm for A5 and 10 nm for A6, were synthesized at the lowest temperatures of 900 and 950 °C, respectively, and still contain unconverted gallium oxide reflected in the O contents of 4.5 and 2.9 wt %. Pyrolyses at 975-1000 °C are required in this method to achieve low O contents below 2 wt % down to a few tenths of a percent, however, at the expense of increased crystallite sizes. Generally, it is difficult with the conventional aerosol-assisted method (generation

(16) (a) Balkas, C. M.; Basceri, C.; Davis, R. F. Powder Diffr. 1995, 10 (4), 266. (b) Paszkowicz, W.; Podsiadło, S.; Minikayev, R. J. Alloys Figure 4. XRD patterns for GaN powders B1-B5 from Ga2O3 nitridation Compd. 2004, 382, 100, and references therein. synthesis. DiVersity of Stability of Nanocrystalline GaN Powders Chem. Mater., Vol. 20, No. 21, 2008 6823

Figure 5. XRD patterns for GaN powders from anaerobic synthesis, C1-C3 (left), and from GaAs nitridation synthesis, D1-D3 (right).

Table 5. Crystalline Phase Analysis for GaN Powders from Anaerobic Synthesis, C1-C3, and from GaAs Nitridation Synthesis, D1-D3 (D (hkl), Average Crystallite Size; a and c, Hexagonal Lattice Parameters; a, Cubic Lattice Constant for Powders D1-D3; u/d, Unable to Determine) C1 (600/16) C2 (700/4/vac) C3 (975/4/vac) exclusive h-GaN (defected): broad broad sharp D (100) (nm) 21 u/d 77 D (101) (nm) 11 40 50 a (Å) 3.193 3.189 3.189 c (Å) 5.197 5.180 5.186

D1 (700/150/slow) D2 (800/90/slow) D3 (800/90/fast) major I; c-GaN (ordered): sharp, 55 wt % v. sharp, 50 wt % v. sharp, 70 wt % D (111), nm 60 33 40 a (Å) 4.513 4.506 4.518 major II; h-GaN (defected): broad, 43.5 wt % sharp, 50 wt % sharp, 30 wt % D (100) (nm) u/d 68 u/d D (101) (nm) n/d 90 u/d a (Å) u/d 3.191 u/d c (Å) u/d 5.190 u/d residual c-GaAs sharp, 1.5 wt % no sharp, traces appear for B4 and B5 because of transient sintering of The XRD scans for powders C1-C3 from the anaerobic particles that results in the high O content of 3.8 wt % in synthesis and for powders D1-D3 from GaAs nitridation B5 after the pyrolysis at 975 °C. This is clearly detected in are shown in Figure 5 and structural data are included in - B5 by XRD in the form of -Ga2O3. It is barely detected in Table 5. The XRD patterns for C1 C3 show typical products B4. from many anaerobic syntheses, namely, phase inhomoge- In general, for all powders from the two already discussed neous GaN which can be best described as crystallites made 18,9 related syntheses, a standard interpretation of their XRD of hexagonal and cubic closed-packed layers. This is spectra suggests the formation of relatively well-crystallized obviously true for nanocrystalline powders C1 and C2 from ° hexagonal gallium nitride as the major product. Only trace the pyrolysis temperatures of 600 and 700 C. At the highest ° quantities of either -Ga O or c-GaN crystalline phases pyrolysis temperature of 975 C the C3 product is only a 2 3 slightly defected h-GaN polytype. Because of peak broadness are sometimes detected. The crystal lattice constants for the and overlapping, the structural data shown in Table 5 for h-GaN polytype are within close proximity with the refer- powders C1 and C2 are only crude estimates of the ence literature data and small discrepancies, as frequently parameters. done in published reports, can be vaguely explained by Powders D1-D3 obtained from nitridation of GaAs are crystallite size dependence in the nanosized range and/or by all found to contain 50% or more of the cubic polytype of structural defects. At the highest applied pyrolysis temper- atures, in most cases h-GaN is the only phase that is (17) (a) Kisailus, D.; Choi, J. H.; Lange, F. F. J. Cryst. Growth 2003, 249, characterized by the best crystalline properties (sharp dif- 106. (b) Lowther, J. E.; Wagner, T.; Kinski, I.; Riedel, R. J. Alloys Compd. 2004, 376, 1. (c) Soignard, E; Machon, D.; McMillan, P. F.; fraction lines, appropriate line intensities vs microcrystalline Dong, J.; Xu, B.; Leinenweber, K. Chem. Mater. 2005, 17, 5465. (d) reference spectrum, crystal lattice parameters very close to Kroll, P. Phys. ReV.B2005, 72, 144407. (e) Ariza, M. J.; Koo, A.; Trodahl, H. J.; Ruck, B. J.; Bittar, A.; Jones, D. J.; Bonnet, B.; Roziere, the ones for microcrystalline GaN, etc.). On the basis of such J.; Moscovici, J. Surf. Interface Anal. 2005, 37, 273. (f) Tran, N. H.; XRD criteria, it is tempting if not common to say that, Lamb, R. N.; Lai, L. J.; Yang, Y. W. J. Phys. Chem. B 2005, 109, 18348. therefore, good quality nanocrystalline GaN has been (18) Hwang, J.-W; Campbell, J. P.; Kozubowski, J.; Hanson, S. A.; Evans, synthesized. J. F.; Gladfelter, W. L. Chem. Mater. 1995, 7, 517. 6824 Chem. Mater., Vol. 20, No. 21, 2008 Drygas et al.

Figure 6. Solid-state 69Ga MAS NMR spectra for GaN powders A1-A8 from aerosol-assisted synthesis.

GaN and the remaining proportion of the defected hexagonal ppm peak, on average 325 ppm, due to mainly second-order phase of GaN (not marked peaks). Powders D1 and D3 also quadrupolar interactions is consistent with high grain crystal- show ca. 1.5 wt % and traces (preferentially grown) of linity coupled with uniformity of the short-range structure unconverted GaAs, respectively, indicating experimental order in the GaN domains responsible for it. A broadened difficulties in full nitridation under the applied conditions. and rather regularly shaped peak at 415-435 ppm contains The c-GaN polytype appears to be a well-ordered cubic phase on the high frequency side an intensity contribution from with the lattice constant a in the range 4.506-4.518 Å the spinning sideband of the 325 ppm peak. When this effect reported for c-GaN19 (e.g., ref 19b: a ) 4.506 Å) and average is accounted for the average value of 420 ppm can be crystallite sizes of 33-60 nm. Because of low intensity and approximated for this high frequency peak. overlapping of peaks, it was not possible to determine with The rather scarce literature NMR data for GaN have been sufficient accuracy similar parameters for the defected interpreted with the notion that the resonance at ca. 325 ppm hexagonal phase in powders D1 and D3. is for the stoichiometric hexagonal GaN polytype while the 69 Solid-State Ga MAS NMR Characterization. The broader peak at ca. 420 ppm has been assigned by some 69 Ga MAS NMR spectra for powders A1-A8 from aerosol- researches either in what appears a rather confusing way to assisted synthesis are shown in Figure 6. Note that the spectra a postulated nonstoichiometric N-deficient phase GaN1-x (0 for powders A1 and A7, in addition to the common < x <1)4b-e or, alternatively, in the well-substantiated reports -100-700 ppm acquisition range, were also acquired in the to a Knight shift due to the presence of conduction electrons frequency windows appropriate for metallic gallium Knight in the semiconductor.6 At first glance, the former interpreta- 10 shift detection. Regarding this, in the high-temperature tion of the higher-frequency peak conforms reasonably well syntheses of these two powders at 1000 °C, traces of metallic with the common knowledge that many high-temperature gallium were observed on the crucible’s bottom upon powder pyrolyzed GaN powders frequently show by elemental discharging. Interestingly, the metallic Ga is clearly detected analysis some nitrogen deficiency, while at the same time by NMR spectroscopy and supported by XPS data (not exhibiting the h-GaN polytype by powder XRD. Following shown) in powder A1 although the relevant XRD spectrum this reasoning and because in the majority of such cases only does not show its presence (Figure 3). Moreover, for powder a single hexagonal pattern for GaN is observed (except for A1 the entire magnetic field range from -100 ppm to the study by Schwenzer and co-workers4d,e mentioned in the +0.49% was probed without success for yet other resonances. Introduction), the concurrent nonstoichiometric nitride phase For these two powders that also display a low intensity broad may have been amorphous in bulk and/or on surface or feature at 100-200 ppm, NMR experiments at variable nanocrystalline in the lowest nanometer range where a long- spinning rates confirmed the feature as the spinning sideband range order as probed by XRD has no substance. Before of the major peak at 320-330 ppm and indicated that the further commenting on this issue, we want to add that our associated sideband contributed to the high frequency side detailed analysis of the XRD spectra for all powders of the second resonance at 420-430 ppm. discussed in this study ruled out significant quantities of a All the spectra show two basic peaks, namely, at 320-330 bulk amorphous phase or a bimodal size distribution that ppm and at 415-435 ppm, both positions observed/uncor- would include large proportions of GaN crystallite fraction rected for the second-order quadrupolar shifts, with charac- in the lowest nanometer range. In the following discussion teristic shapes and widely varied relative intensities. The of the high frequency peak at 420 ppm, we will invoke either relative sharpness and apparent asymmetry of the 320-330 an idea of the decomposition-promoted nonstoichiometric N-deficient GaN phase (rather, dopant-stabilized defects of (19) (a) Purdy, A. P. Chem. Mater. 1999, 11, 1648. (b) Jegier, J. A.; this kind) or the Knight shift involvement whenever they McKernan, S.; Purdy, A. P.; Gladfelter, W. L. Chem. Mater. 2000, 12, 1003, and references therein. (c) Jouet, R. J.; Purdy, A. P.; Wells, are deemed as consistent with experimental data and both R. L.; Janik, J. F. J. Cluster Sci. 2002, 13 (4), 469. approaches will later be critically confronted. DiVersity of Stability of Nanocrystalline GaN Powders Chem. Mater., Vol. 20, No. 21, 2008 6825

Figure 7. Solid-state 69Ga MAS NMR spectra and XRD patterns for annealed/partially decomposed GaN powders A4 and A8.

In the NMR spectra for powders A1 and A3, an additional here result from the applied high pyrolysis temperatures and small intensity peak at ca. 357 ppm is seen and can be can be compared with the appropriate values for the assigned to small quantities of c-GaN.5 Some cubic GaN is previously discussed powders A7 and A8 that, contrary to also confirmed in these powders by detailed analysis of the powders A1-A4, show the prevailing NMR resonances at relevant XRD patterns. 325 ppm. Each of the powders A5-A8 shows the prevailing peak To directly address the question of progressing GaN at 325 ppm and only a trace peak at 420 ppm suggesting decomposition in relation to the evolution of the high that these materials are mostly composed of the crystalline frequency peak at 420 ppm, the following experiment was and stoichiometric h-GaN. This finds a reasonable explana- carried out. Powders A4 (both NMR peaks of significant tion from the way they were prepared in relation to GaN intensities) and A8 (prevailing NMR peak at 325 ppm) were stability at the temperatures of pyrolysis. Namely, powders additionally annealed under a flow of NH3 at 1000 °C for A5 and A6 were pyrolyzed for6hattherelatively low 3 h followed by 1050 °C for 5 h, i.e., under conditions forcing temperatures of 900 and 950 °C, respectively, under which noticeable decomposition of GaN and darkening of the conditions gallium nitride shows a decent thermal stability, materials. Figure 7 includes the comparative 69Ga MAS and therefore, the amounts of the presumed decomposition- NMR spectra (left) and the XRD scans and derived hexago- derived N-deficient phase would be rather small. Completely nal lattice parameters/crystallite sizes (right) for the annealed different mechanism may have operated for powder A8 that powders. The NMR spectra for both annealed powders was prepared at 975 °C, 6 h, but was later washed/cleaned clearly show increased proportions of the 420 ppm peak at with an HF solution. On the one hand, these pyrolysis the expense of the 325 ppm peak. At the same time, the conditions are rough enough to initiate a noticeable GaN hexagonal polytype persisted and refined while the average decomposition but, on the other hand, washing with a very crystallite sizes in both cases increased relative to the initial intrusive HF solution could have preferentially dissolved/ powders (compare these XRD data with those in Table 3). removed the compositionally defective nitride phase. It is Undoubtedly, slowly progressing decomposition of GaN worth mentioning that in the first paper reporting the specifics forms a basis for the evolution of the high frequency NMR of both the 69Ga NMR and 71Ga NMR spectroscopy for GaN peak at 420 ppm while, simultaneously, the XRD long-range powders including the broad peak at ca. 450 ppm for the order improved characteristics appear to result from con- then-proposed (undefined) impurity, its intensity significantly comitant grain growth/recrystallization of the GaN particles. 4a decreased upon washing the powder with aqua regia. It is The NMR spectra for powders B1-B5 from the Ga2O3 also instructive to point out that powders A5-A8 are all nitridation synthesis are displayed in Figure 8. The specific characterized with the XRD patterns each showing well- feature here is the high frequency side resonance shifted crystallized h-GaN of differing average crystallite sizes slightly toward higher frequencies within the 420-450 ppm (Figure 3, Table 3). range and at least part of this shift could be attributed to the In powders A1-A4, the peak at 420 ppm is significant if peak’s overlapping with the spinning sideband of the 325 not prevailing in relation to the peak at 325 ppm that would ppm peak. In the series of closely related powders B1-B3, support copious amounts of the N-deficient phase, especially only powder B1 from the lowest pyrolysis temperature of in powder A2. On the other hand, there is nothing outstanding 900 °C has a predominant 325 ppm peak. Powders B2 and in these powders’ XRD spectra: they all show patterns for B3 from the higher pyrolysis temperatures show increased ordered h-GaN. The relatively high average crystallite sizes proportions of the 420 ppm peak and 450 ppm peak, 6826 Chem. Mater., Vol. 20, No. 21, 2008 Drygas et al.

nitridation. A quantitative comparison of the relative intensi- ties in these standard spectra is rather equivocal since the effective tilting angle in the NMR experiment is different for the hexagonal and cubic phases. A relatively narrow line at 215 ppm4a,5b indicates some unconverted GaAs in powder D1 and merely traces of it in powders D2 and D3. This is in a qualitative agreement with the relevant XRD patterns although no GaAs at all is detected by XRD in powder D2 (Figure 5). It is appropriate to recall at this point that these powders are composite mixtures of two major components, namely, cubic and hexagonal GaN polytypes (Table 5). The best quality powder D2 (ca. 50/50 of c-GaN/h-GaN) displays a complex NMR spectrum with four overlapping resonances in the nitride range. On the lower-frequency side, there are two medium intensity and relatively sharp peaks at 325 and 357 ppm that can be linked to the stoichiometric h-GaN and c-GaN polytypes, respectively. The very broad peak centered at 420 ppm can be associated with the presumed N-deficient phase originating from hexagonal GaN, Figure 8. Solid-state 69Ga MAS NMR spectra for GaN powders B1-B5 as discussed earlier. However, the strongest and sharper peak from Ga2O3 nitridation synthesis. at ca. 470 ppm is a new feature in this study. By analogy respectively, while their XRD scans display better crystal- with the pair of peaks at 325 and 420 ppm linked to the linity than for powder B1, thus confirming a similar trend hexagonal polytype, we tentatively suggest that the peak at as observed among powders from the aerosol-assisted 470 ppm reflects yet another N-deficient phase derived this method. It appears that the application of different pyrolysis time from cubic GaN. It is interesting to refer now to this times in these two chemically similar methods causes faster powder thermal stability by TGA/DTA as the decomposition ° recrystallization and decomposition phenomena in the Ga O temperature of 1040 C (Table 2). The powder was synthe- 2 3 ° nitridation system (12 h) than in the related aerosol-assisted sized at 800 C, i.e., much below its decomposition tem- synthesis (6 h) as can clearly be seen from comparison of perature; but despite this, the NMR spectrum shows signifi- the NMR and XRD spectra for powders A6 and B2 cant amounts of the presumed N-deficient phases. This can pyrolyzed at the same temperature of 950 °C. be understood in terms of the extremely long pyrolysis time The left-hand part of Figure 9 contains the 69Ga MAS of 90 h to be compared with the nonequilibrated conditions ° NMR spectra for powders C1-C3 from the anaerobic of fast heating (10 C/min) in the TGA/DTA experiment. It synthesis. Powders C1 and C2 from the lowest pyrolysis appears, therefore, that nanosized GaN powders can undergo ° temperatures of 600 and 700 °C, respectively, are each significant changes already at 800 C if sufficiently long characterized by a single NMR peak at 325 ppm (stoichio- heating times are applied. metric h-GaN). On the other hand, the NMR spectrum for The NMR spectrum for powder D1 shows an asymmetric powder C3 pyrolyzed at 975 °C is dominated by a strong peak at 325 ppm (stoichiometric h-GaN) and a broad and and broad peak at 420 ppm complemented by a weak peak slightly asymmetric (due to a spinning sideband presence) at 325 ppm. The specific feature of the anaerobic synthesis line on the high frequency side at 470 ppm (N-deficient phase in comparison to both the aerosol-assisted and Ga2O3 from c-GaN) with no significant intensity around 420 ppm nitridation syntheses is a possibility to prepare nanopowders (N-deficient phase from h-GaN). In this regard, this powder of GaN at significantly lower temperatures while of structur- was synthesized at the lowest temperature of 700 °C, much ally defected hexagonal polytype.9,18 The XRD patterns for below GaN decomposition temperatures for these materials. powders C1 and C2 show the broad diffractions typical for Interestingly, the NMR spectrum suggests that under these a strongly defected h-GaN, whereas for C3, the pattern is conditions (700 °C, 150 h) the crystalline h-GaN is more typical for only a slightly defected h-GaN polytype (Figure stable than the crystalline c-GaN because the peak at 470 5, Table 5). These interpretations are in good agreement with ppm assigned to the N-deficient phase from decomposition the available TGA stability data (Table 2). The thermal of c-GaN and the peak at 325 ppm for the stoichiometric decomposition of powder C2 takes place at 930 °C, much h-GaN dominate. higher than the 700 °C pyrolysis temperature for the powder, Powder D3 was synthesized under similar conditions as and therefore, no clear signs for the presumed N-deficient powder D2 differing mainly by a significantly higher phase from h-GaN decomposition are observed by NMR. ammonia flow during the synthesis of the former. Interest- On the other hand, powder C3 was prepared at 975 °C close ingly, this results in the NMR spectrum for D3 showing to the 1050 °C decomposition temperature of this material, almost exclusively the band of lines for the N-deficient i.e., temperature high enough to have resulted in significant phases with the major peak at 470 ppm and a shoulder at N-deficient phase formation. ca. 420-430 ppm and only traces of lines if any for the The right-hand part of Figure 9 shows the 69Ga MAS stoichiometric phases. This suggests that under the applied NMR spectra for powders D1-D3 prepared via GaAs pyrolysis conditions, cubic GaN was preferentially formed DiVersity of Stability of Nanocrystalline GaN Powders Chem. Mater., Vol. 20, No. 21, 2008 6827

Figure 9. Solid-state 69Ga MAS NMR spectra for GaN powders from anaerobic synthesis, C1-C3 (left) and from GaAs nitridation synthesis, D1-D3 (right). and later significantly decomposed. This is further supported frequency shift of the gallium NMR line by some 160 ppm by XRD that confirms the prevailing c-GaN. from ca. 40 ppm (6-coordinate) to 200 ppm (4-coordinate)20a N-Deficiency-Related Defects vs Knight Shift Effect and similar shifts are observed for various 6-, 5-, and in GaN Powders. The only apparent correlation observed 4-coordinated gallium-oxygen compounds.20b,c However, in this study among the GaN powders between the specifics a quantitative evaluation of the relative proportions of the of the XRD patterns and NMR spectra is that the high Ga nuclei linked to the high-frequency NMR peak in relation temperature promoted better crystallinity of the given poly- to the characteristics of the related XRD data as well as the type is accompanied by increased proportions of the NMR fact that the chemically unlikely 3-coordinated gallium peak appearing in the 420-450 ppm range for h-GaN or at centers would almost certainly have an enormous quadrupole ca. 470 ppm for c-GaN, somewhat counter-intuitively if the coupling constant and hence broaden a resulting NMR shifts would have to be associated with distinct decomposi- resonance beyond detection rule out a simple structural tion phases. At the same time, as already mentioned, a careful N-deficient Ga(3N)-center effect, i.e., 3-N-coordinated Ga analysis of the XRD spectra shows no signs of significant chemical environments to be directly responsible for the high- proportions of a bulk amorphous phase and/or nanometric frequency peak. There must be instead another factor phase in the lowest nanosized range that could be related to crucially contributing to the shift while related to the the additional distinct phase implied by NMR in any of the N-deficiency. samples. We want to point out, however, that the powders We think that our observations can be reasonably ac- constitute a size-heterogeneous system of which particle counted for under two restrictions. First, from the composi- sizes, crystalline properties, and surface vs core character- tion/structure point of view, we have to assume that the istics are averaged out by powder XRD data. Therefore, the proposed by others and consistent with this study high observed bimodal NMR response for each polytype can be temperature-induced sites in GaN such as some stable non- linked, for instance, to different particle sizes/size ranges or Ga(4N) centers, e.g., 4-coordinated Ga(3N)(X), where X is to particle surface vs core domains, each case yielding under a stabilizing foreign atom (likely, X ) O or H), are in certain circumstances a single XRD pattern. sufficiently low quantity and random distribution throughout Considering again the assignments of the gallium NMR the bulk of GaN crystallites or in surface layers as to not spectra, the low frequency ca. 325 and 357 ppm line positions influence significantly the stability and long-range order of for h-GaN and c-GaN, respectively, seem to be well- the GaN lattice as substantiated by the XRD data. Some documented and accepted for a range of GaN materials in potential in addressing this problem in a quantitative way the sense that they are related to four-coordinated gallium may reside in calculations of properties and energetic stability Ga(4N) chemical environments as expected in both stoichio- of various native (compositional) defects in GaN.21 Recent metric polytypes. The assignment of the corresponding peaks data of this kind support, for instance, a preferential formation at 420-450 ppm (h-GaN) and 470 ppm (c-GaN), i.e., of N-defects, their increased concentration and mobility with significantly shifted toward higher frequencies by some 100 ppm or more vs. the respective stoichiometric phase peaks, (20) (a) Massiot, D.; Farnan, I.; Gautier, N.; Trumeau, D.; Trokiner, A.; at first glance, can misleadingly suggest the existence of some Coutures, J.- P. Solid State Nucl. Magn. Reson. 1995, 4, 241. (b) Bradley, S. M.; Howe, R. F.; Kydd, R. A. Magn. Reson. Chem. 1993, stable three-coordinated Ga(3N) domains resulting from 31, 883. (c) Ash, J. T.; Grandinetti, P. J. Magn. Reson. Chem. 2006, partial decomposition with nitrogen depletion. For instance, 44, 823. (21) For example, see (a) Ganchenkova, M. G.; Nieminen, R. M. Phys. the decrease in gallium coordination number from 6 to 4 in ReV. Lett. 2006, 96, 196402, and references therein. (b) Van de Walle, two different environments in -Ga2O3 causes a high C. G.; Neugebauer, J. J. Appl. Phys. 2004, 95 (8), 3851. 6828 Chem. Mater., Vol. 20, No. 21, 2008 Drygas et al. temperature, and localized nature of associated electrons offer an invaluable insight into, apparently, a gradual within the radius encompassing the second nearest transition from the stoichiometric with no conduction neighbors,21a whereas others stress the importance of con- electrons (exclusive 325 ppm peak for h-GaN or 357 ppm taminant oxygen or hydrogen atoms on defect formation and peak for c-GaN) to the nonstoichiometric with conduction stabilization.22,23 As a matter of fact, defect stabilization by electrons (exclusive 420-450 ppm peak for h-GaN or 470 oxygen seems to be highly probable here given the nature ppm peak for c-GaN), whereas well-crystallized pure GaN of the applied precursors (O-bearing gallium compounds in powders from high-temperature/long-time treatments. A more two syntheses) and common ammonia reactant contamina- detailed analysis of peak broadening and variations in peak tions (traces of water vapor and oxygen). The inferred from positions will be necessary to relate the Knight shift evolution several studies gradual decomposition of GaN with nitrogen to the particle size and/or particle surface vs core nonuni- depletion at appropriately high temperatures/long heating formities in the powders. times could thus be associated with the formation of 4-coordinated Ga(3N)(O) defects (oxygen incorporation into Conclusions the GaN lattice). In this regard, the powders prepared at lower temperatures contain on average smaller crystallites (XRD) Nineteen GaN powders prepared by four synthesis meth- of apparently highly stoichiometric composition reflected in ods constituted a comprehensive materials stage for the study the predominant Ga(4N) chemical environments (NMR). of short/long-range structural and electronic properties related Higher synthesis/annealing temperatures, especially, coupled to the powders’ thermal stabilities. These materials showed with extended treatment times result in larger and better- by XRD diffraction crystalline GaN polytypes encompassing ordered (XRD) while more defected (NMR) crystallites, the the single ordered or defected hexagonal GaN and composite latter phenomenon enhanced by the nitride’s lattice instability mixtures of the ordered cubic and defected hexagonal GaN and recrystallization/crystal growth phenomena. to yield, with increasing pyrolysis or annealing temperatures, Second, and crucially, we have to accept that crystallites materials of better crystallinity and larger average crystallite or domains with such native compositional defects exhibit sizes. At the same time, no significant amounts of amorphous unique electronic properties with a specific gallium NMR or nanocrystalline GaN domains in the lowest nanosized response. In this regard, a plausible shallow donor oxygen range were detected while metallic gallium was occasionally dopant in such defects could be a source of conduction observed. The 69Ga MAS NMR spectroscopy for the powders electrons and resultant new “electronic phase”.22,23 We would provided two peaks with varying relative intensities for each like to recall that similar kind of the electronic phase GaN polytype. The high-frequency shifted peak was related associated with a specific Knight shift in semiconductors due to the decomposition-induced N-deficient domains and to conduction electrons in both intentionally (e.g., 0.13 wt dominated over the low frequency peak for stoichiometric % Ge) and unintentionally doped GaN has recently been GaN after higher pyrolysis temperatures and/or extended proposed and well substantiated by Yesinowski et al.5a,6 As pyrolysis times. The XRD-derived features had a general a matter of fact, the 71Ga MAS NMR spectrum for their Ge- correlation with the corresponding NMR data in that the doped GaN powder resembles closely our NMR spectra for better crystallinity corresponded with the evolution and powders A3 (Figure 6) or C3 (Figure 9) by showing the increased proportions of the NMR high frequency peak. This prevailing peak at ca. 440 ppm (Knight shift effect) and a trend could be satisfactorily accounted for by assuming that smaller intensity and sharper peak at 330 ppm (stoichiometric the rather scarce N-deficient sites (e.g., oxygen dopant h-GaN).6b The similarity of their NMR spectra to our results stabilized defects or native N-defects) resulting from partial suggests that the specific electronic phase and associated decomposition of GaN form a structural basis for the conduction electrons are intrinsically common in many if electronic phase with conduction electrons, the latter causing not all high-temperature-synthesized or annealed intentionally the Knight shift effect in gallium NMR and, therefore, pure GaN powders. significantly altering many materials properties. The knowl- In summary, it seems coherent to directly link the edge of the trends in formation and proportions of the two formation of the thermally induced foreign atom stabilized different phases, both being the function of powder thermal N-deficient domains/native N-vacancies in the pure GaN stability in relation to the temperature/time history of nanopowders with the presence of conduction electrons synthesis or annealing, will be crucial in appraising GaN responsible for the Knight shift effect in the NMR spectra. powder suitability for many high-temperature applications. From a different angle, the NMR spectroscopy in combina- tion with XRD measurements and TGA/DTA stability data Acknowledgment. J.F.J. acknowledges the support of the Polish Ministry of Science and Higher Education/AGH-UST, (22) For example, see (a) Kikkawa, S.; Nagasaka, K.; Takeda, T.; Bailey, M.; Sakurai, T.; Miyamoto, Y. J. Solid State Chem. 2007, 180, 1984. Grant 11.11.210.120. R.T.P. acknowledges support from the (b) Wright, A. F. J. Appl. Phys. 2005, 98 (10), 103531. (c) Wright, U.S. National Science Foundation (Grant CHE-9983205) for A. F.; Seager, C. H.; Myers, S. M.; Koleske, D. D.; Allerman, A. A. the U.S.-Polish collaboration. J. Appl. Phys. 2003, 94 (4), 2311. (23) Sheu, J. K.; Chi, G. C. J. Phys.: Condens. Matter 2002, 14, R657. CM800645Q